For an image formation by way of a photographical technique, there has been employed a photosensitive composition comprising a photosensitive component such as diazonium salts, azides or quinonediazides, or a photo hardening composition comprising a photocrosslinkable component. Most of these compositions are sensitive to ultraviolet light but not sensitive to visible light. This is a problem to be solved because it is desired that printing is carried out by exposing with visible light in lieu of ultraviolet.
Scanning exposure techniques with laser beams have recently progressed so that it should be taken into consideration that these scanning exposure techniques are applied to imaging techniques. Especially, the use of a visible light laser, such as an argon laser, is more desired because of the aspect of energy and equipment cost. Accordingly, a composition more sensitive to visible light is strongly desired.
Japanese Patent Publication No. 114139/1982 and Japanese Patent Application Ser. No. 170599/1985 disclose a photopolymerizable composition sensitive to a visible light having a wave length of 400 to 700 nm, wherein a dyestuff is combined with a photosensitizer. These photopolymerizable compositions are still insufficient in sensitivity to visible light and therefore, when applied to an imaging technique using laser scanning system, require a high output power laser which costs much. Accordingly, further improvement of sensitivity is required.
It has been found that a kind of xanthene or thioxanthene dyestuff is very sensitive to visible light when combined with a polymerizable monomer, a photosensitizer and an organic peroxide. It also has been found that a cured composition exibits excellent physical properties for the use mentioned in the field of the invention.